In the initial sighter milk trials, it was realised that the steam temperature did not correlate with the saturated steam pressure. To confirm this in the water trial, step changes in the steam pressure were made to see their effect on the steam temperature. The steam control valve was manually opened to step increase the steam pressure and the steam temperature was monitored. Theoretically it was expected that the steam temperature would increase by the equivalent amount to the steam pressure and remain stable at the saturated steam conditions pertaining.
Figure 5-5 shows differential pressure on the primary Y-axis and differential temperature on the secondary Y-axis.
Figure 5-5: Steam differential temperature profile due to change in differential steam pressure in Effect B
The steam temperature seemed to overshoot from its saturated conditions with every step change in the steam pressure. The overshoot in the steam temperature shows that the steam was not saturated and that superheat was entering the calandria. Superheated steam makes the evaporation process very inefficient because the heat transfer coefficient for condensing saturated steam is many times more than the heat transfer coefficient for superheated steam. Hence, it was necessary to resolve this issue before the evaporator could be used for the milk trials. The data shown in Figure 5-5 is from
RE Water Trial 4 8 12 16 4 5 6 Time (hrs) Pr e s s u re ( k Pa ) 4 8 12 16 Te m p e ra tu re (Deg re e s ) Pressure Temperature
effect B only but similar trends were seen in effects A and C during random times in the run length. Effects D and E were relatively good in their performance with respect to de-superheating.
The de-superheat system of the RE for all effects is as shown in the Figure 5-6 below.
Figure 5-6: De-superheat system on Research Evaporator
Steam enters the steam control valve at 220 kPa (abs) from the low pressure steam
header. Pressure is reduced to 40 kPa (abs) by the steam control valve. Water at 20 oC is
sprayed into the incoming steam to fully saturate it. A small cyclone is employed to remove any excess water added during de-superheating. Excess water is flushed down through a steam trap to the condenser temperature and is extracted by the vacuum pump.
To ensure that the de-superheating system was functioning properly and all the steam entering the calandria was saturated, the amount of energy required to saturate the steam was calculated and compared to the amount of energy supplied by the water supply to
de-superheat the steam. The enthalpies (kJ.kg-1) were converted to equivalent volumes
of water supplied and as shown in the bar chart in Figure 5-7.
Superheated Steam
Water Spray
Figure 5-7: Water supply for de-superheating system
• The energy required to de-superheat the steam was calculated for the steam flow
rate when the steam valve was completely open. The maximum flow rate was calculated based on the steam pressure and the steam pipe diameter.
• The minimum water requirements were calculated from the assumption that all
of the water supplied at 20 oC would evaporate and that the enthalpy input to the
water during sensible heating and evaporation would be used to cool down the superheated steam. On the other hand, for maximum water requirements, it was assumed none of the supplied water would evaporate and that all of the energy lost from the steam would be used for sensible heating of the water.
• The actual water flow rate from the de-superheat nozzles was measured at two
different water pressures.
Abundant water was being supplied to Effects D and E but the amount of water supplied to Effects A, B and C while close to the minimum water requirement was still sufficient to de-superheat the system. When the de-superheating water line was taken down to the floor, it was realised that there was a leak in the line at effect A as shown in the Figure 5-8. ͲǤͲ ͳͲǤͲ ʹͲǤͲ ͵ͲǤͲ ͶͲǤͲ
Water Flow Rate (ml.s-1)
Ev ap orator Calandria Effect ͲͲ
Figure 5-8: Leak in de-superheat line
All of the manual valves were rusted and many small rust elements were blocking the spray nozzles. Due to these blocks, a full cone spray pattern was not being achieved.
Figure 5-9: Rusted parts of the de-superheating line and the incomplete hollow cone spray nozzle The plant was re-tested after the leak was fixed; the pipe cleaned and rusted parts replaced. The problem still remained unresolved.
Water flow rates were initially measured at 600 kPa (maximum pressure in the line was 650 kPa) and it was commonly seen that the water pressure would go down drastically in the pilot plant when the water from the same line was used in other equipment. Hence, higher flow rates needed to be achieved using the same water pressure to remain above the minimum water requirements. This could be done by increasing the spray nozzle diameter or replacing the hollow cone pattern spray nozzles to the full cone pattern.
Figure 5-10: Hollow cone spray nozzle type on the left and full cone spray nozzle type on the right. The cone pattern was changed to obtain higher water flow rates. The change finally helped in resolving the de-superheating water issue and the steam temperature obtained after testing corresponded very well with the steam pressure.